1. Field of the Invention
The present invention relates to a light emitting display. More particularly, the present invention relates to an Organic Light Emitting Diode (OLED) display using the electro-luminescence of an organic material.
2. Description of the Related Art
In general, an Organic Light Emitting Diode (OLED) display electrically excites a fluorescent organic material to emit light and visualize an image by voltage programming or current programming N×M organic light emitting cells.
Such organic light emitting cells are called Organic Light Emitting Diodes (OLEDs) because they have diode characteristics. An OLED includes anode (ITO), organic thin film, and cathode (metal) layers. The organic thin film has a multi-layer structure, including an EMitting Layer (EML), an Electron Transport Layer (ETL), and a Hole Transport Layer (HTL), so as to balance electrons and holes and thereby enhance light emitting efficiency. Furthermore, the organic thin film separately includes an Electron Injection Layer (EIL) and a Hole Injection Layer (HIL). The N×M organic light emitting cells arranged in a matrix format form an OLED display panel.
A pixel of N×M pixels (i.e., a pixel positioned at the first column and the first row) of an active matrix OLED display includes subpixels and these subpixels respectively include OLEDs emitting red (R), green (G), and blue (B) light. In a striped structure of subpixels, the subpixels are coupled to different data lines, and are coupled to a common scan line.
A driving transistor is arranged between a source voltage and an anode of the the red OLED and transmits a current for light emission to the red OLED. A cathode of the red OLED is coupled to a voltage VSS that is lower than the source voltage. A current passing through the driving transistor is controlled by a data voltage supplied by a switching transistor. A capacitor is coupled to a source and a gate of the driving transistor and sustains a supplied voltage. A gate of the driving transistor is coupled to the scan line through which a scan signal containing on/off information is transmitted, and a source of the driving transistor is coupled to a data line through which a data voltage for the red subpixel is transmitted.
A data voltage VDATA from the red data line is supplied to the gate of the red driving transistor when the switching transistor is turned on in response to a selection signal supplied to the gate of the transistor. A current IOLED flows to the red transistor, the current corresponding to the voltage VGS between the gate and the source and stored by the capacitor, and the red OLED emission corresponds to the current IOLED. The current IOLED flowing through the red OLED is given by Equation 1 as follows.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                                                                V                      TH                                                                                          )                            2                                                          Equation        ⁢                                  ⁢        1            
As shown by Equation 1, the current corresponding to the supplied data voltage is supplied to the red OLED, and the red OLED emits light with a brightness corresponding to the supplied current.
The supplied data voltage has multiple-stage values within a predetermined range so as to display gray scales.
As described above, in the OLED display, one pixel includes subpixels, and each subpixel includes a driving transistor, a switching transistor, and a capacitor for driving an OLED. A data line for transmitting data signals and a source line for transmitting the source voltage are formed on each subpixel. Accordingly, problems of line arrangement in a pixel area and aperture ratio reduction in a light emitting area occur since more lines are required to drive the pixel. Accordingly, an arrangement having a reduced number of lines and elements to drive the pixels is needed.